This invention relates to improving adhesion of metal surfaces, such as copper to an insulating layer, in the manufacture of printed circuit boards.
A multilayer circuit board (MLB) has, among other things, a number of metal layers defining circuit patterns, and a number of insulating layers there-between. The metal layers defining circuit patterns today are typically formed from copper, and the insulating layers are typically formed from a resinous fiber-impregnated dielectric material. These respective layers can have a wide variety of thickness. For example, they can be on the order of only microns thick, or much thicker.
In manufacturing MLBs it is desirable to enhance the adhesion between the conducting and insulating layers to avoid delamination in subsequent manufacturing operations or in service. So called “black oxide” processes had been used for years which created a strongly adherent copper oxide layer to which an insulating layer would adhere better. Black oxide processes have, for most of the industry, been replaced by processes such as described in U.S. Pat. No. 5,800,859 involving formation of an organometallic conversion coating (OMCC). These OMCC processes involve exposing the copper circuit layer to an adhesion promotion solution, which contains various components including an oxidizer, an inhibitor, and a mineral acid.
One limitation on OMCC processes has been that after processing a number of boards through the same solution, sludge can form in the solution as copper builds up. The effective life of a solution is therefore limited by its tolerance for loading of copper. Another limitation is that the OMCC must be a uniform color, such as, for example, a dark brown or chocolate color. The industry associates this color with a uniform coating which has strong adhesion properties. A dark uniform color is preferred because it provides color contrast with copper to aid in inspection for defects. For example, it provides contrast for inspection for the so-called “pink-ring” defect. OMCC processes which produce significantly lighter coatings are generally unacceptable, or at least undesirable for most applications. For a lighter coating, “pink ring” defects are substantially more difficult to detect.